This invention relates to noise detection and reduction when electronic converters/transformers or magnetic transformers are used together with lamp dimmers.
As is known lamp dimers reduce the power fed to a lamp by reducing the average voltage applied to the lamp. To this end, two types of dimmer are known: leading edge dimmers that switch the voltage on after the zero crossing point in the AC cycle; and trailing edge dimmers that switch the voltage off before the zero crossing point in the AC cycle. Typically, lamp dimmers employ very fast triacs or thyristors such that the rate of switch-on in a leading edge dimmer and the rate of switch-off in a trailing edge dimmer is substantially instantaneous, typically 15-25 xcexcs.
One of the known problems with dimmers, be they trailing or leading edge types, is that they may give rise to acoustic noise, and this noise is exacerbated when the dimmer is used in conjunction with a transformer, particularly magnetic transformers or electronic converters/transformers providing DC output. The noise may be at least partially caused by the vibration of the ferromagnetic laminates forming the core of the choke/s (in dimmer and transformer) and/or the vibration of certain capacitors. It is also known that the actual lamp filaments are caused to vibrate, and this is the case particularly with DC output electronic converters/-transformers, giving rise to an additional source of acoustic noise or hum. This noise/hum is disturbing and unpleasant and it is obviously desirable to reduce it as much as possible.
There exist both magnetic and electronic transformers on the market. Electronic transformers (or xe2x80x9cconvertersxe2x80x9d) may be AC or DC (input and) output units. In the case of DC output transformers, a chopper is used to convert the DC voltage to a pulsating voltage, which may then be transformed to a different voltage, and ultimately rectified and smoothed. Regardless of the dimming technology used, there is always inherent noise associated with a dimmer/transformer combination that cannot be avoided, and the problem is particularly acute with DC output transformers.
Prior art approaches require additional components to be installed in the dimmer itself in order to reduce noise. Thus, in the case of leading-edge dimmers, the prior art uses a de-buzzing coil (choke), which is bulky, expensive, reduces efficiency and must be tuned to the dimmer circuitry. Coil selection can be a time-consuming task.
For trailing edge dimmers, designed for use with electronic transformers/-converters, the conventional noise reduction solution is to add a large capacitor for reducing the rate of decline of the trailing edge. When used in conjunction with an electronic transformer/converter, the capacitor may be placed before or after the diode bridge. However, if such a modified transformer is used with a leading edge dimmer, it actually increases the noise. Thus, known electronic lamp transformers/-converters employing a capacitor as explained above are intended for use with trailing edge dimmers only.
This means that it has not so far been possible to take a commercially available leading edge dimmer and use it in conjunction with a lamp transformer without avoiding noise, unless a xe2x80x9cdebuzzing coilxe2x80x9d is selected and used. Conversely, common practice for noiseless dimming with the use of electronic transformers has been the use of electronic transformers incorporating a capacitor as described above, in conjunction with trailing edge dimmers.
JP 6089784 published Mar. 29, 1994 describes a low noise dimmer that attempts to reduce noise by smoothing a supply voltage waveform by controlling rise and fall curves of the lamp voltage. To this end, prior to the supply voltage crossing to zero from the negative half cycle, a forward power switch element is turned on, and an output voltage is detected. When an effective voltage approaches a target value, a microcomputer enters a fall motion and gradually lowers the output voltage within a predetermined time to the zero line smoothly. Also, for the negative side voltage, a reverse side power switch element and the microcomputer perform the same operation so that a rise motion is smooth. Thereby, hum noise production is prevented without applying an electromagnetic surge to an incandescent lamp.
This patent appears to relate to the known problem of switching a triac or thyristor during zero crossing of the AC voltage, since triacs are known to stop conducting when the current is zero and require a trigger signal to initiate conduction, such conduction being possible only if the anode voltage of the triac is larger than the cathode voltage thereof. Therefore, two switching devices are employed and are controlled to conduct during opposite halves of the AC cycle. The switching between the two thyristors itself causes noise and JP 6089784 appears to relate to a smoother mechanism for effecting the required switching so as to reduce the acoustic noise.
U.S. Pat. No. 5,319,301 issued Jun. 7, 1994 to Callahan M. et al. discloses an inductor-less light dimmer with semiconductor power devices coupled between an alternating current supply and a lamp load. Undesirable effects of the high current demands of cold lamp filaments are reduced by initially increasing the conductive portion of half-cycles, relative to the proportion required to produce the desired amount of average power, while avoiding transitions at phase angles that would produce excessive losses. A transition shape may be employed in this mode and in normal operation that maximizes audible lamp noise suppression for a given level of thermal losses.
WO 91/06047 published May 2, 1991 to Bayview Technology Group, Inc. and entitled xe2x80x9cReverse phase control switching circuit and method without zero crossing detectionxe2x80x9d discloses a method and circuit, in a dimmer, for reverse phase control of alternating current being delivered to a load wherein voltage-controlled semiconductor switches such as MOSFET""s and IGBT""s are used as electronic switches to conduct voltage during the leading edge of the AC voltage cycle and conduction is terminated when the desired phase angle of the current flow has been reached. The disclosed method and circuit eliminate the need for zero crossing detection of the AC waveform and ensure that the voltage-controlled switches are always turned on before the zero crossing thereby minimizing radiated interference and incandescent lamp hum.
Both U.S. Pat. No. 5,319,301 and WO 91/06047 appear to relate to leading edge dimmers only, to a noise solution inside the dimmer itself only, and appear only to address problems associated with switching at zero crossing of the AC voltage so as to reduce lamp hum.
It would therefore be desirable to provide a universal electronic lamp transformer that may be used with any commercially available dimmer, be it a leading edge or a trailing edge type, without requiring modification to the dimmer or the addition of external components in order to reduce noise, and also to provide a module, with the same dimming noise reduction effect, that may be added to conventional magnetic transformers.
It is an object of the invention to provide a lamp transformer (or standalone module) and method that are suitable for use with both leading and trailing edge dimmers so as to reduce lamp hum.
According to one aspect of the invention, there is provided a method for reducing acoustic noise produced during use of a dimmer, the method comprising:
(a) detecting that a dimmer is present, and if so:
i) detecting whether the dimmer is a leading edge dimmer or a trailing edge dimmer,
ii) if the dimmer is a leading edge dimmer, reducing the rate of rise of the leading edge, and
iii) if the dimmer is a trailing edge dimmer, reducing the rate of fall of the trailing edge.
According to a further aspect of the invention, there is provided a controller for reducing acoustic noise produced during use of a leading edge dimmer, the controller comprising:
a leading edge controller responsive to an input voltage fed thereto for producing a control signal upon detection of a leading edge, and
a linear switch coupled to the leading edge controller and responsive to the control signal for linearly switching the input voltage so that a rate of rise of the leading edge is decreased.
Preferably, such a controller is adapted for reducing acoustic noise produced during use of a trailing edge electronic dimmer, and further comprises:
a leading-trailing edge detector responsive to an input voltage fed thereto for detecting whether the input voltage results from a leading edge dimmer or a trailing edge dimmer, and
a trailing edge controller coupled to the leading-trailing edge detector and responsive to detection of a trailing edge dimmer for disabling the leading edge controller and decreasing a rate of decline of the trailing edge of the input voltage;
said leading edge controller being coupled to the leading-trailing edge detector and responsive to detection of a leading edge dimmer for disabling the trailing edge controller.